1. Field of the Invention
The present invention generally relates to electronic test circuits and, more particularly, to testing microelectronics at wafer level.
2. Background Description
The test of state of the art microelectronics at wafer level often requires the application of structural test patterns to the Device Under Test (DUT). In a structural test, the DUT is loaded with an input pattern of test vectors, a single clock cycle is applied, and the resultant output vector is read out. Often this test sequence is repeated many times during the course of the test, in rapid succession. When the DUT contains a large number of transistors, this test sequence invariably produces a voltage noise pulse on the power planes of the probe card used to bias the DUT. The noise spike is produced by momentary increase in current drawn by the chip during the test cycle. This noise spike cannot be fully compensated by the on chip decoupling capacitor network. Depending on the total number of structural test cycles, the DUT may draw its full operating current over a time scale from nanoseconds (ns) to hundreds of milliseconds (ms) which can produce power supply voltage transients with these durations. These voltage noise spikes (which momentarily reduce the power supply voltage applied to the DUT) can cause the DUT to fail the applied test pattern if the voltage reduction is severe enough, event though the DUT is defect free. It should be pointed out that, because of the transient nature of structural test patterns, it is impossible to eliminate these voltage noise spikes by the application of conditioning cycles prior to the test. To mitigate the effects of voltage noise spikes, hardware test engineers have traditionally employed several different solutions, depending on the duration of the spike. The power supply transients associated with DUT test can be broadly categorized into three time scales:
Time scales in the 0.1 to 100 millisecond range are generally within the response times of DC power supplies, connected in conventional ways. However, as the current requirements of the DUT have increased, and voltage levels have decreased, even these longer time voltage transients have become more difficult to tame, because they have become larger in amplitude. To address power spikes in this time scale range the trend has been to employ advanced, high performance power supplies (often called Point of Load or POL supply) with faster response times, located closer to the DUT. However, even with the best POL power supply electronics, practical considerations (the regulation units are physically large and hence cannot be located on the probe card, the probe card/power supply interface contains significant inductive parasitics, etc.) make the fastest response times of these units 5 microseconds or greater.
Voltage noise spikes of shorter duration, from Ins to 10 microseconds (10 μs) are generally compensated by decoupling capacitor networks mounted on the probe card. The capacitors are mounted as close to the DUT as is practical, to reduce the response time of the bypass network to the voltage noise spike. However, as the level of integration of silicon circuits has increased, voltage supply levels have decreased and current requirements have increased. Increasing DUT supply current requirements in combination with a fixed probe card area for the mounting of decoupling capacitors is making it impossible to provide enough bypass capacitance to adequately reduce voltage noise spikes in the 1 ns to 10 μs time range when testing state of the art components with very high supply currents (˜100 amps). Adding more capacitance farther away from the DUT is not effective for reducing high frequency noise spikes because the time constant of the bypass network becomes too great. The size of capacitors is limited too; using capacitors with larger capacitance values in the same area is ineffective if the larger capacitors have larger Equivalent Series Inductance (ESL) or Equivalent Series Resistance (ESR) values. Although many different technologies are available for capacitors, one rule generally holds true: as technologies are employed with a higher capacitance to volume ratio (specific capacitance), ESL and ESR increase, making them less effective at filtering the higher frequency voltage spikes. This restricts the capacitors used to filter transients in the Ins to 10 μs range to high performance, low ESR and ESL types with low specific capacitance values, which limits the total capacitance of the network because of the limited space available on the probe card close to the DUT.
Voltage spikes of duration under ins are typically effectively filtered by on chip bypass capacitance networks.
There is a large body of literature on power bus noise reduction and active noise filtering. Some relevant references are listed below:    Ang, M., Salem, R., and Taylor T, “An On Chip Voltage Regulator Using Switched Decoupling Capacitors”, ISSCC 200, WP27.7, pp. 438–439 (2000)    Mobin, S., and Shu, K., “Power Delivery Challenges of High Power Logic Device at Sort”, Presented at Soutwest Test Workshop, June 2004